What shape is a spirit level bubble?

[Michael GilliganParticipant @michaelgilligan61133]

A useful reference, Nick

I’m not even going to try answering the original question, as I would be way out of my depth … but I do have a comment:

The minimal bubble, in a plain straight tube, would [in that zenith view] present as something very close to a circle, and if it were somehow elongated would present as a ‘racetrack oval’ with semicircular ends BUT as Nick’s photo nicely illustrates, the curvature of the vial distorts those semicircles.

It therefore seems reasonable to assume that the bubble in a sensitive level will be less distorted than one  in a general-purpose level.

My maths and 3D Geometry are both too weak to usefully develop that thought

<blush>

MichaelG.

[Michael Gilligan]

On Michael Gilligan Said:

…
It therefore seems reasonable to assume that the bubble in a sensitive level will be less distorted than one  in a general-purpose level.

My maths and 3D Geometry are both too weak to usefully develop that thought

<blush>

MichaelG.

I’ve been pondering that too!  And wishing my 3D-CAD and maths skills were much better.

Comparing Nick’s vial with my model, it’s seen that his real bubble is much rounder than my simplistic model:

I suggest surface tension is a good thing in that it conveniently rounds the real bubble into a tighter easier to see shape.   The liquid often contains dye, and possibly it’s engineered to optimise the shape too.   (The surface tension of water is reduced by adding washing up liquid.)

My guess is that the bubble in a barrel-ground sensitive level is distorted in much the same way as a cheap bent tube provided both are on a level plane.  I think the problem with bent tubes is that they distort the bubble when tilted sideways, making them less accurate.  Barrel-ground would be more tolerant of side tilt, I feel, but this is pure conjecture!

Dave

[DC31kParticipant @dc31k]

Barrel-ground would be more tolerant of side tilt…

Barrel ground can also be used truthfully upside down. Think of a magnetic Stabila scaffolder’s level that only has one reference edge. Scaffolders do not carry two levels, one for use on top of the tube and one underneath.

Even though I only understand about one word in ten, the paper above is worth reading. There are two factors in play: buoyancy and surface tension. The inside of the tube puts geometrical boundary conditions on the surface that touches it.

In that paper, the authors use a finite element program called ‘surface evolver’ to do their work. If you go and look at the surface evolver homepage, the program’s author starts with a catenoid – the shape that a soap bubble takes up when suspended between two parallel rings. The page also shows some interesting skeletal structures for 3D printing.

[Michael GilliganParticipant @michaelgilligan61133]

Just in case it’s not intuitively obvious to everyone … the minimal bubble “au naturel” would be spherical: all re-shaping of that pure form must therefore be attributable to the characteristics of the vial.

MichaelG.

[DC31k]

On DC31k Said:

Barrel-ground would be more tolerant of side tilt…

Barrel ground can also be used truthfully upside down. Think of a magnetic Stabila scaffolder’s level that only has one reference edge. Scaffolders do not carry two levels, one for use on top of the tube and one underneath.

Even though I only understand about one word in ten, the paper above is worth reading. There are two factors in play: buoyancy and surface tension. The inside of the tube puts geometrical boundary conditions on the surface that touches it.

In that paper, the authors use a finite element program called ‘surface evolver’ to do their work. If you go and look at the surface evolver homepage, the program’s author starts with a catenoid – the shape that a soap bubble takes up when suspended between two parallel rings. The page also shows some interesting skeletal structures for 3D printing.

Oh you don’t want to complicate things by introducing magnetic fields. Contact angle (wetting) change in diamagnetic materials like water alcohol and acetone so will change the shape of the bubble. ;O)

I like catenoid, that really helps to describe it.

The earlier reference to ME article on making vials was a rod held at one end and bent down under screw control. With the tube threaded onto the rod and a slurry of grinding material the vial was brought to a barrel shape by running it up and down the rod and gradually rotating to distribute the grinding action around the tube.

Polishing was achieved by succession of finer grits.

regards Martin

 

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Dave, this photo isn’t directly over the bubble, but I think it shows that there is no real discernible difference in the shape of the bubble, when the vial is tilted to one side, and watching it while turning a full rotation, I couldn’t see it change shape.

Regards Nick.

[Author]

Posts